The Internet Storm Center published a graph showing historic trends for the "Survival Time" of unpatched, unprotected (windows) computers connected to the internet. Turns out, this number dropped from about 40 minutes last year, to 20 minutes this year. The survival time is calculated as the average time between reports for an average target IP address. If you are assuming that most of these reports are generated by worms that attempt to propagate, an unpatched system would be infected by such a probe. The data is collected from a large number of networks with different types of upstream protection. So if you are on an unprotected cable/DSL line, you may see probes much more frequently. Either way, 20 minutes is not long enough to download patches. The Honeynet Project did publish a paper with some stats back in 2001.
This seems a bit extreme. Did they mean a machine running a server? I find it hard to believe a unpatched Win 98 machine would die that fast. I'll have to run a test someday.
It does explain why Microsoft is pushing XP SP2.
Buy a Mac.